Current limiting fuse including improved fuse element

ABSTRACT

A current limiting fuse including an improved fuse element. The fuse element or link includes portions or regions of reduced cross-section which melt during overload to form a series of voltage arclets. The fusible material of the fuse element between notched regions of reduced cross-section is uniquely shaped to take advantage of electrical potential distribution within it, and the natural tendency of electrical current to flow from notch to notch in specified paths of least resistance. The fuse link has a predetermined geometric shape to accommodate the flow of current in an efficient manner and to eliminate those portions of the fuse link which conduct little or no electrical current during normal operation.

[ Dec. 25, 1973 CURRENT LTMITING FUSE INCLUDING IMPROVED FUSE ELEMENT[75] Inventors: Donald D. Blewitt, Pittsburgh, Pa.;

David L. Ayers, West Lafayette, Ind.

[73] Assignee: Westinghouse Electric Corporation,

Pittsburgh, Pa.

221 Filed: May 18, 1972 21 Appl. No.: 254,598

[52] US. Cl. 337/295, 337/159 2,259,053 10/1941 Xenis .,337/295X PrimaryExaminer -Bemard A. Gilheany Assistant ExaminerF. E. Bell Attorney-A. T.Stratton et a1.

[57] ABSTRACT A current limiting fuse including an improved fuseelement. The fuse element or link includes portions or regions ofreduced cross-section which melt during overload to form a series ofvoltage arclets. The fusible material of the fuse element betweennotched re gions of reduced cross-section is uniquely shaped to takeadvantage of electrical potential distribution within it, and thenatural tendency of electrical cur rent to flow from notch to notch inspecified paths of least resistance. The fuse link has a predeterminedgeometric shape to accommodate the flow of current in an efficientmanner and to eliminate those portions of the fuse link which conductlittle or no electrical current during normal operation.

5 Claims, 9 Drawing Figures PATENIEUusces I975 v SHEET 10F 3 PRlOR ARTPRIOR ART FIG.2

Heb

PRIOR ART FIG. 3

PRIOR ART 1 CURRENT LIMITING FUSE INCLUDING IMPROVED FUSE ELEMENTBACKGROUND OF THE INVENTION This invention relates to electrical currentlimiting current limiting fuses may be of the cartridge type and mayinclude elongated sections or elements of fusible material or linksbetween electrically conducting end ferrules. The fuse material may beenclosed in an insulating casing which is disposed between and attachedto the conducting end ferrules and which may surround a pulverulent, arcquenching material such as quartz sand which, in turn, may completely orpartially enclose or surround the previously mentioned fuse element orelements. The fuse elements may be notched or otherwise reduced incross-section at specific points along their. length. This is especiallytrue where the fuse is of the higher voltage type wherein a plurality ofserially spaced arclets is desired to be established upon blowing of thefuse. Often the fuse material is ribbon shaped, or it has a very smallthickness compared with its width and length. Generally the fuse elementincludes one or more regions of limited cross-section to establish andmaintain one or more corresponding electrical arcs for a predeterminedperiod of time to limit the current which may attempt to continue toflow through the fuse at the time of fusing. In one type of fuseelement, triangularly shaped notches are provided which may besymmetrical about the longitudinal center line of the fuse element. Inanother fuse, elements having square notches may be used. In otherinstances, circular or curved sections may be removed from the innerportion of the fuse element. A well known geometric configuration is thetriangularly notched configuration mentioned above. Along the length ofthe fusible material, sections of fuse material having thecharacteristic shape of an isosceles triangle are' removed in a nearapex-toapex relationship, symmetrical about the longitudinal centerline. This may be done at fixed intervals along the length of thefusible material. The resultant fuse element may appear to be of aribbon shape having periodically placed or axially spaced conductingsections which are joined by small notches. The apex of the isoscelestriangles may be truncated so that the notched section may be slightlyelongated. The reduction in cross-section of the fusible materialincreases the resistance of the ribbon-like fuse material. As thecurrent flow is traced along the length of the fuse it can be seen thatcurrent density is relatively higher in the regions of the previouslymentioned notches and lower in the regions of larger cross-section.Since current flows from a notched area into an area of largercross-section otherwise known as the heat dissipating regions of thefusible material, it would be advantageous and useful to change theshape of the fuse element in the area of larger cross-section to conformto the actual path of the current into and through that region.

SUMMARY OF THE INVENTION In accordance with the invention a currentlimiting fuse comprises a fuse element or link having a plurality ofareas of reduced cross-section and a unique geometric shape betweensuccessive areas of reduced cross section to more efficiently conductelectrical current flowing through the fuse element. The resultant fuseelement has a gently curving or arcuate shape between spaced portions ofreduced cross-section which starts at a narrow restricted neck or notchthen flows smoothly or gradually outwardly to a bulge or larger sectionintermediate the adjacent pair of notches and then gradually convergesto the next notch of the fuse element. Corners which may be evident onknown elements are eliminated because they conduct very little of thetotal current flowing between the notches compared with the central bodyof the fuse link or element. The resistance and heat dissipatingqualities of the fuse element are not affected or reduced appreciably.

BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of theinvention reference may be had to the preferred embodiments exemplary ofthe invention shown in the accompanying drawings in which:

FIG. 1 shows a prior art current limiting fuse element with a knownshape;

FIG. 2 is an enlarged view ofa portion of the fuse element shown in FIG.1;

FIG. 3 is a further enlarged view of a portion of the fuse element shownin FIG. 2;

FIG. 4 shows lines of equal electrical potential in a section or portionof the fuse element shown in FIGS. 1, 2 and 3;

FIG. 5 shows a resistance-analog equivalent circuit of the fuse elementshown in FIG. 4;

FIG. 6 shows a geometric representation of the plotting of a currentstreamline in a fuse element;

FIG. 7 shows current streamline distribution in a fuse element;

FIG. 8 shows a fuse link with constant potential lines and constantcurrent stramlines superimposed; and

FIG. 9 shows a cartridge type current limiting fuse with contoured fuseelement in accordance with the teachings of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to the drawingsand FIG. 1 in particular, a prior art fuse element 10 is shown. Fuseelement 10 may be made from a ribbon of silver or silver-alloy material12 having triangular sections removed, forming areas of reducedcross-section such as 14 and 16. The triangular portions removed may beisosceles in shape. At any instant of time, electrical current such as imay flow from left to right or vice versa through fuse element 10. Thedistribution of the electrical current i in the region of largercross-section 17 for example, of fuse element 12 is different than thedistribution of the current i in the areas of reduced cross-section ornotched areas such as 14 and 16. Current i may be direct or alternatingelectrical current and flow in either left to right or right to leftdirections.

Referring now to FIG. 2, a longitudinal side elevation of a rectangularfuse element 10 is shown in enlarged form. In this'case a longitudinalcenter line or axis 18 is shown-and threelines which are perpendicularto the longitudinal. center line 1.8 are also shown. These are line 20,line 22 and line-24. Notched-sections 14 and 16 are shown aswell as twolarger current carrying fuse element portions or sections 17. Line 20may be generally equidistant from notch 14v and notch 16 and isgenerally perpendicular to the longitudinal center line 18. Line 20 isshown on fuse link section 19a. The adjacent fuse link or fuse elementsection 19b has a line 22 generally perpendicular to center line 18 andgenerally equidistant from notch 16 and notch 16a. Line 24 substantiallybisects the notched region 16 and intersects center line 18 at rightangles. Referring to intermediate fuse section 19a, it can be seen thatcenter line 18 and perpendicular or normal line 20 form a coordinatesystem within fuse link 19a having four generally equal andsubstantially symmetrical, geometric regions numbered I, II, III and IV.It will be noted that the region indicated by III, that is, the regionto the right of perpendicular line 20 and above center line 18 iscrosshatched in the drawings.

Referring now to FIG. 3, an expanded or further enlarg'ed view of linkportions 19a and 19b in fuse element is shown in the region of notch 16.Perpendicular center line 24 and longitudinal line 18 are also shown/Itshould be noted that notched section 16 has a relatively flat outersurface having a length indicated as 2d (d+d). This is slightlydifferent from the overall view shown in FIG. 2 where notch 16 is shownto be formed from the removal of material shaped like two isoscelestriangles disposed generally apex-to-apex and oppositely disposed fromeach other. As can be seen by referring again to FIG. 2, only the upperleft portion of the notched region 16 is shown, and that is the portionwhich corresponds to region III shown in FIG. 2.

It has been found that electrical current, such as i flowing in fusesection 10 does not distribute itself evenly and homogeneouslythroughout the entire current carrying material 19a, but rather flows ina manner similar to current inan electrical conductor which is under theinfluence of skin effect. The exact distribution of current in a fuselink or in a section of fuse link such as 19a, may be calculated by thefollowing method. First, assume that a two dimensional solution ofcurrent distribution is satisfactory for deriving current streamlines orelectrical potential lines, that is, assume that the currentdistribution normal to the plane of FIG. 4 is uniform. Second, solveLaplaces equation for potential in a section of current limiting fuse,Laplace's equation being WEI Lag. Said in another way, the secondpartial derivative of the potential V with respect to X (thelongitudinal direction in the fuse element) plus, the second partialderivative of the potential V with respect to Y (the direction normal'tocenter line 18 in the fuse element) must equal zero. I is perpendicularto center line 18 and X is parallel to center line 18. The resultingequations are as follows:

( m y) a v/zs x o The equation for electrical potential is then solvedin the traditional manner such as with the use of boundary conditions toaid in the solution.

Referring now to FIG. 4, the fuse link portion 19a as shown in FIG. 2 isshown in enlarged form-Concentrating on region III, it will be .notedthat region III is bounded by a perimeter comprised ofa slanted ordiagonal line 32, a horizontal line 30, the vertical line 34, thehorizontal center line 18 and the vertical line 24. These peripherallines may be useful to establish the boundary conditions for solution ofequations (1) and (2). The first condition which must be met is that nocurrent must flow out of the periphery of the fuse link, that is, nocurrent may flow through peripheral lines 30 or 32 in region III. Thesecond boundary condition is that distribution of electrical potentialmust be a mirror image of itself about the center line 18. This meansthat there will be no flow of current across center line 18. This is onereason why a relatively small but well defined region may be useful tofind potential distribution for the entire fuse section or link 19a. Thethird boundary condition is that a constant potential line equidistantbetween two notches such as 16 and 14 and perpendicular to center line18 will be an isopotential line. Line 34 meets this requirement. Thefourth condition is that a line perpendicular to center line 18 in thecenter of the notched'region or in region 16 and equidistant from thetwo ends of the notches or at a distance d from point 23 as shown inFIG. 3 will also be an isopotential line, at some potential otherthanthat of line 34. Examination will show that perpendicular line 24meets this requirement. Using these four boundary conditions the Laplaceequation may be solved producing a solution for electrical potential.The solution is: a constant C, times X plus the sum from N l'to infinityof the quantity C,, times the hyperbolic sine of the subquantity N 1rX/L times the cosine of the subquantity N 1r l/L N in this case is aninteger, X is the distance from the intersection of lines 18 and 34 orthe lower left corner of region III in a direction from left to right, Yis the vertical direction from the intersections of lines 18 and 34upward, and C,, and C, are constants which are determined by applicationof the previously named boundary conditions. L, is the longitudinaldistance between perpendicular lines 34 and 24 and L is the verticaldistance between longitudinal center line 18 and periphery line 30. Theequation may be written thusly: Y

The above equation is a closed-form solution for potential distributionin a model of a notched current limiting fuse with the added criterionbeing that an orthogonal coordinate system must be used. A plot ofequipotential, (V constant), distribution lines within region III isshown in FIG. 4. In this case five equielectricalpotential lines areshown at 34, 36, 38, 40 and 42. It will be noted that equipotentiallines 34 and 36 meet the boundary conditions of being perpendicular tocenter line 18 and straight. Since the distribution of potential regionIII is similar to regions I, II and IV, the equipotential lines may bedrawn throughout the entire fuse link 19a as shown by equipotentiallines 44, 46, 48 and 50 in addition to those previously mentioned. The

V=C X+ 2 0,, sinh cos Cauchy-Riemann criterion is now used to arrive atan The Cauchy-Riemann criterion states that a relationship existsbetween potential V and a stream function I such that:

or, in terms of the stream function from Equation (4):

At any point X the total current flowing in the X direction, that is inthe direction of the longitudinal center line 18, between thelongitudinal center line 18 and any point Y, is given by where T is thethickness of the element ribbon, normal to the coordinates X and Y. FromEquations (6) and (8) it follows that av T (W I c From Equation (9) itcan be seeii that a definite relationship exists between the streamfunction I at point X, Y in the fuse element and the total currentflowing in the X direction between longitudinal center line 18 and thepoint X, Y. The total current flowing in the X direction between thelongitudinal center line 18 and periphery. lines 30 and 32 is:

Now a nondimensional stream function can be defined From Equations (9),(l0), and (l 1) it can be seen that the nondimensional stream function W(X, Y) is the fraction of the total current'flowing in the direction ofthe longitudinal center line 18 which flows between the longitudinalcenter line l8'and point X, Y. Likewise, Equations (9), l0), and l lshow that the nondimensional stream function I (X, Y), or currentfraction, v

can be determined mathematically from Equation (3), which is amathematical expression for potential V at point X, Y.

Referring now to FIG. 6, a portion of fuse element 10 or link portion19a, region lll is shown. Center line 18 forms the bottom-most portionof the fuse element portion shown. Here two constant potential lines VC, and V= C are shown. At some point P, having coordinates X, Y, on theconstant potential line V C the nondimensional stream function I has avalue of K where 0 s K s 1. Likewise on the constant potential line V= Cthere is a point 0 which has the same value of nondimensional streamfunction K. By considering many such lines of constant potential, apoint on each such line can t& found where the nondimensional streamfunction 1 equals K. By drawing a line through these points of constantnondimensional stream function T= K a current streamline is defined asshown in FIG. 6. At each point on this line the nondimensional streamfunction I has the value K. By definition of the nondimensional streamfunction, at every point along the streamline W K the fraction of thetotal current flowing in the direction of the longitudinal center line18 which flows betv en the longitudinal center line 18 and thestreamline I K is K. Thus no current flows across the streamline T K.

Referring now to FIG. 8, a plot of nondimensional stream functionequicurrent streamline I i s shown. The longitudinal center line 18 hasa value of I O and the peripheral boundries 30 and 32 have anondimensional stream function value of"F= l.O. Referring back toEquation (3), since the hyperbolic sine function is odd and the cosinefunction in the mathematical definition of potential V is even, then Vat a point (X, l) equals Vat point (X, Y) and V at a point (--X, Y)equals -V at point (X, Y). That is, the distribution of potential V inregion IV is a mirror image about the longitudinal center line 18 of thedistribution of potential V in region Ill. Likewise the distribution ofpotential V in region l is a mirror image about the perpendicular centerline 20 of the distribution of potential V in region ill with the signchanged and potential V in region ll is a mirror image about line 34 ofthe potential V in W with the sign changed. it follows from theCauchy-Riemann criterion that the inverse of these mirror image lawsapply to the nondimensional stream function 1 Thus FIG. 8 shows lines ofconstant potential and streamlines for the entire link 19a.

It will be noted that a psi normal (1) equals 0.5 plot isdepicted inFIG. 7. This means that 50 percent of the total current flows betweenthe psi normal 1 equal -0.5 curve and the center line 18. This isrepresented by the hatched or shaded area 27.

Referring now to FlG. 8, a composite plot of normalized equicurrentstreamlines in a complete fuse link 19a is shown. The potential linesare represented by lines 34, 38, 40, 42, 46, 48 and 50, and thestreamlines are represented by 1 equal 0, T equal 0.25, V equal 0.5, Iequal 0.75 and 1 equal 1. An important aspect of the invention mayberecognized by'realizing that the area enclosed between the two shown psinormal equals 0.75 lines represents a significant portion of theconducting'link 19a and by also noticing that percent of the entireelectrical current flowing between notch l-6-and notch 14 must flowwithin the psi normal equals 0.75, lines adjacent to the top and bottomof link portion 19a. Consequently, all of the electrical material abovethe psi normal equals 0.75 line in the top part of section or link 19aand below the psi normal equals 0.75 line in the bottom part of fuselink portion or section 19a may be removed and 75 percent of the currentwill still flow in the remaining conducting material of the fuse link19a with the same potential applied across the fuse. Of course, thisconstruction increases the electrical resistance in element section 19afrom R to R/0.75 1.33R. but does not increase it in the same proportionto the decrease in current carrying capacity. For example, although, notshown, a W equals 0.95 curve may be superimposed on conductor linkportion 19a andthat part of the conductor link outside or not includedwithin the region described by the psi normal equals 0.95 line mayberemoved and 95 percent of the electrical current will continue to flow,in fuse link 19a with the same electrical potential applied across thefuse. However, the removed portion constitutes a rather significantphysical amount of the fuse link portion 190. In summary, this meansthat most of the current flowing in a tapered or notched fuse portionsuch as 19a as shown in FIG. 8 flows in the center of the fuse elementwhen viewed from the side so that the corners or edges may be eliminatedproviding a slight increase in resistance to the fuse element or linkportion 19a but providing a larger significant decrease in the amount ofmaterial used to form the fuse link or element without a significantreduction in the current carrying or conducting capacity, simply becausea s ignificant amount of current does not flow in the peripheral, outerportion of the fuse link portion 19a.

Referring now to FIG. 9, a cartridge type current limiting fusestructure 58 embodying a fuse link or fuse element 60 is shown havingregionsof reduced crosssection such asl4 and 16, the periphery 76 ofwhich is determined by the criteria previously described. Line 76 mayhave the, shape of an electrical current streamline which may becalculated through the use of the Cauchy-Riemann criterion and theLaplace equations for potential in the fuse link 60. It will be notedthat fuse structure 58 as shown in FIG. 9 has end ferrules 64 and 66, atubular insulating member or housing 68 disposed between the endferrules or electrical conducting members 64 and 66, and enclosing arcquenching pulverulent material 70 which may be quartz sand. The fuselink or element 60 which may be connected and disposed between oneelectrically conducting interface member 72 which is adjacent to bothone end of the fuse link 60 and the electrically conducting ferrule 64and a similar interface means near ferrule 66 not shown. Current maythen flow through the electrical fuse 58. However, a significant amountof fusible material in fuse link or element 60 has been removed prior toconstruction. I

It is to be understood that the previously described use of currentstreamline construction to determine the exact shape of current limitingfuses having areas of reduced cross-section may be determined by othermethods than the computer oriented mathematical approach previouslydescribed. The use of trial and error is proper as isthe use of plottingpotential distribution on resistive paper. It is also to be understoodthat as long as an orthogonal coordinate system may be developed for afuse link, that any shape of reduced crosssection including squareshaped or rectangularly shaped notched sections or even circularlynotched sections may be used. It is also to be understood that thisinvention need not be applied only to cartridge type current limitingfuses nor need it be applied only to current limiting fuses havingsingle fuse elements.

The apparatus embodying the teachings of this invention has severaladvantages for example, an advantage lies in the more efficient use ofafuse element in a current limiting fuse to carrying the maximum amountof current in any given cross-section of material without usingunnecessary material which carries very little current. An overallreduction in fuse link size and a corresponding reduction in fusestructure are also advantages.

We claim as our invention:

1. An electrical fuse comprising spaced terminals and a fuse element offusible material, said fuse element being electrically connected to andinterposed between said spaced terminals, the dimensions of a planarportion of said fuse element being definable in a twodimensionalorthogonal coordinate system, at least a portion of the peripheral shapeof said planar portion of the said fuse element being definedby thenondimensional stream function, wherein said nondimensional streamfunction is less than 1 and greater than zero.

2. An electrical current limiting fuse comprising a pair ofspaced'terminals, a fuse element formed from fusible material, said fuseelementbeing electrically connected and interposed between saidterminals, said fuse element having at least one region of reducedcross-section, the dimensions of a planar portion'of said fuse elementbeing definable in a two-dimensional orthogonal coordinate system, atleast a part of the outer periphery of said planar portion of saidfusible material, not including a substantial portion of any said regionof reduced cross-section, being defined by the nondimensional streamfunction, wherein said nondimensional stream function is less than 1 andgreater than zero.

3. The combination as claimed in claim 2 comprising pulverulent arcquenching material disposed proximate to at least a portion of saidfusible material, a tubular outer shell of insulating material, saidouter shell being disposed between said terminals and attached theretoto form an enclosure around said fuse element and said are quenchingmaterial 4. The combination as claimed in claim 3 wherein said fusiblematerial comprises a silver based alloy and said are quenching materialcomprises quartz sand.

5. The combination as claimed in claim 4 wherein said non-dimensionalstream function has a value in a range greater than zero but less thanor equal to 0.95.

1. An electrical fuse comprising spaced terminals and a fuse element offusible material, said fuse element being electrically connected to andinterposed between said spaced terminals, the dimensions of a planarportion of said fuse element being definable in a two-dimensionalorthogonal coordinate system, at least a portion of the peripheral shapeof said planar portion of the said fuse element being defined by thenon-dimensional stream function, wherein said non-dimensional streamfunction is less than 1 and greater than zero.
 2. An electrical currentlimiting fuse comprising a pair of spaced terminals, a fuse elementformed from fusible material, said fuse element being electricallyconnected and interposed between said terminals, said fuse elementhaving at least one region of reduced cross-section, the dimensions of aplanar portion of said fuse element being definable in a two-dimensionalorthogonal coordinate system, at least a part of the outer periphery ofsaid planar portion of said fusible material, not including asubstantial portion of any said region of reduced cross-section, beingdefined by the non-dimensional stream function, wherein saidnon-dimensional stream function is less than 1 and greater than zero. 3.The combination as claimed in claim 2 comprising pulverulent arcquenching material disposed proximate to at least a portion of saidfusible material, a tubular outer shell of insulating material, saidouter shell being disposed between said terminals and attached theretoto form an enclosure around said fuse element and said arc quenchingmaterial
 4. The combination as claimed in claim 3 wherein said fusiblematerial comprises a silver based alloy and said arc quenching materialcomprises quartz sand.
 5. The combination as claimed in claim 4 whereinsaid non-dimensional stream function has a value in a range greater thanzero but less than or equal to 0.95.